Resistive random access memory (ReRAM) technologies are being proposed as a replacement for many semiconductor memory applications. These technologies can potentially enable the manufacture of non-volatile memories that are higher density, lower power, and faster access than technologies now in use.
ReRAM comprises arrays of non-volatile memory elements each of which in turn comprise a volume of material that exhibits bistable resistance. The resistance state can be repeatedly switched back and forth between high and low values using modest write/erase voltages and power levels, and the resistance state can be read with lower voltage levels that do not change the state. Bistable resistance is typically created in a layer of material that normally functions as an insulator at typical operating voltages. Filamentary conduction paths can be formed through sufficiently thin layers by aligning chains of defects which persist after forming. Typically, there is a one-time forming step requiring a relatively high voltage and current. Thereafter, the filamentary conduction paths can be reformed (“set”) and broken (“reset”) at one set of voltage and current levels and read (without changing state) at a much lower voltage and current.
Bistable resistance has been demonstrated in near-stoichiometric thin layers of many transition metal oxides including oxides of tantalum, niobium, hafnium, aluminum, titanium, and lanthanum among others. The conduction paths in these oxides are formed by aligning defects in the form of oxygen atoms vacancies. By using layers having a thickness of less than about 100 Å, conductive filaments can be formed through an otherwise insulative material using modest set voltages. Moreover, these filaments can be reformed and broken using appropriate set and rest voltages. Detailed understanding and precise control of the formation of these conductive filaments remains an area of active research at this time. Improved device characteristics can be made both with precise control over the material stoichiometry and by additional dopants that can facilitate the reversible alignment of defects in very thin layers.
Memory arrays based on ReRAM memory cells can be made as simple cross-bar structures wherein a layer of bistable resistive material is sandwiched between crossed parallel electrodes. Actual memory architectures must work with the available voltages from supporting read/write/erase circuitry, and it is often advantageous to add a current-limiting resistor at each memory element location. These current-limiting resistors can be created advantageously by adding a second layer of a fixed resistive material adjacent to the bistable resistive material forming the memory elements. For example, Lee, et al. (Nature Materials 10, 625-630, doi:10.1038/nmat3070, 2011) describe ReRAM devices based on bilayers of material based on Ta2O5. A resistive layer (resistivity 107-108 Ωcm) is created using dc reactive ion sputtering of Ta metal in an Ar/O2 atmosphere, where the O2 level is adjusted to create a substoichiometric TaO2.5-x layer having sufficient oxygen deficit to provide permanent, but limited, conduction paths. Lee reports that a near-stoichiometric (1013 Ωcm) thin layer was created on top of the resistive layer by exposure to an oxygen plasma. This thin surface layer reportedly had the required bistable resistance to create memory elements. The total thickness of the oxide layers was 30-40 nm (300-400 Å) of which 10 nm (100 Å) was the near-stoichiometric top layer.
However, the relatively thick structures disclosed by Lee would limit the areal density of a ReRAM memory array based on the devices. Typically, such a layer can be patterned with features no smaller than the layer thickness. What is needed are methods of making similar and smaller structures comprising layers of transition metal oxides of variable composition with precise control over the composition and thickness of each layer.